Internal combustion engines with an exhaust emission control system utilizing a catalyst generally control the mixture ratio of air to fuel in an air-fuel mixture combusted in the internal combustion engine, that is, the air-fuel ratio, in order to allow the catalyst to efficiently remove toxic components of exhaust gas for purification. The air-fuel ratio is typically detected by an air-fuel ratio sensor provided in an exhaust passage in the internal combustion engine and feedback-controlled by controlling the amount of fuel injection so as to make the air-fuel ratio equal to a predetermined target air-fuel ratio.
A typical configuration adopted to detect the air-fuel ratio includes an A/F sensor installed on an upstream side of an exhaust emission control catalyst to provide an output generally proportional to the air-fuel ratio and an O2 sensor installed on a downstream side of the emission exhaust catalyst to provide an output that changes rapidly when the air-fuel ratio changes across a stoichiometric value. This configuration typically performs main feedback control controlling the fuel supply amount based on the output value from the A/F sensor so as to make the exhaust air-fuel ratio equal to the target air-fuel ratio and sub feedback control allowing correction of the fuel supply amount using a correction amount set based on the output value from the O2 sensor. The purpose of performing the two types of feedback control is to use the output from the O2 sensor to correct the output from the A/F sensor, the latter being likely to be erroneous as a result of insufficient mixture of exhaust gas or thermal degradation of a detection element.
Moreover, in order to reduce the amount of time needed for the sub feedback control utilizing the output from the O2 sensor, a control method called learning control has been proposed which involves calculating and holding a learning value corresponding to a constant deviation between the output value from the O2 sensor and the actual exhaust air-fuel ratio and correcting the fuel supply amount based on the learning value (see, for example, Patent Literature 1). The learning value of the learning control is, for example, calculated so as to incorporate at least a part of the correction amount of the sub feedback control. Such a configuration allows the output from the A/F sensor to be quickly corrected utilizing the learning value, for example, even immediately after the internal combustion engine is restarted, when the output from the A/F sensor has not been sufficiently corrected under the sub feedback control.
A possible failure such as element cracking in the O2 sensor precludes appropriate detection from being continued, and is desirable to be detected on board. The O2 sensor generally exhibits a low output in a lean atmosphere. However, possible element cracking results in a difference in gas concentration between an element inside area exposed to the outside air and an element outside area exposed to exhaust gas. Thus, the output voltage of the O2 sensor decreases to provide an output apparently indicative of a lean state. Therefore, the sensor can be determined to be subjected to element cracking when, in spite of an increase in the amount of fuel injection, the output value from the O2 sensor is leaner than a predetermined value lasts for more than a predetermined time (see, for example, Patent Literature 2). In order to suppress degradation of emission until the sensor is determined to be subjected to element cracking and during the period of retreat travelling following the determination, Patent Literature 2 further implements correction amount guard control allowing adjustment of the correction amount for air-fuel ratio control for the sub feedback control by setting a limit on the correction amount for the air-fuel ratio control according to the distribution of the output value from the O2 sensor.
On the other hand, when, for example, a failure occurs in fuel injection systems for some cylinders to significantly vary the air-fuel ratio among the cylinders, the exhaust emission is disadvantageously degraded. Such a significant variation in air-fuel ratio as degrades the exhaust emission is desirably detected as abnormality. In particular, for automotive internal combustion engines, onboard detection of inter-cylinder air-fuel ratio imbalance has been demanded in order to prevent a vehicle with degraded exhaust emission from traveling. In recent years, attempts have been made to legally regulate the onboard detection of inter-cylinder air-fuel ratio imbalance.
To accomplish this purpose, various configurations have been proposed which detect inter-cylinder air-fuel ratio imbalance based on an output from an A/F sensor provided on the upstream side of a catalyst. For example, with focus placed on an extreme increase in the amount of hydrogen in exhaust observed when the air-fuel ratio shifts to a rich side in some cylinders and on removal of the hydrogen from the exhaust for purification using the catalyst, an apparatus described in Patent Literature 3 detects inter-cylinder air-fuel ratio imbalance based on the state of a deviation between a detection value from the A/F sensor provided on the upstream side of the catalyst and a detection value from an O2 sensor provided on the downstream side of the catalyst. The configuration determines the presence of inter-cylinder air-fuel ratio imbalance when the detection value from the O2 sensor deviates significantly toward a lean side with respect to the detection value from the A/F sensor.